Muscle determination and differentiation is being intensely studied in several vertebrates including rats, mice, chickens, and more recently, amphibians. Amphibians are especially advantageous because even the earliest stages of muscle cell induction can be studied in vivo, in large numbers of synchronously developing embryos. For this reason, the role of growth factors and of determining genes in muscle development is well under way in amphibians. In addition, the entire set of embryonic and larval muscles changes at metamorphosis, under the control of thyroxine. Therefore, amphibians also provide a premier example for studying thyroid control of muscle development and function. In all these experiments, the ability to identify muscle fiber types is crucial. Surprisingly, however, the investigators simply do not know how many kinds of muscle fibers exist in early amphibian embryos and larvae, nor precisely when they develop, not whether fibers can switch types, particularly at metamorphosis. Studies of muscle cell induction, differentiation, and regulation are all limited until the investigators understand better the pattern of fiber type expression. As part of a long term goal of understanding how embryos control the spatial distribution of muscle fiber differentiation, the formation of embryonic muscle fiber types will be studied in the frog Xenopus laevis, focussing on three specific questions. First, how many skeletal and cardiac myosin heavy chains are expressed in embryos and larvae? cDNAs for two have been cloned and there is evidence for two or three more. These additional cDNAs will be cloned by rescreening extant libraries and by cloning from RNA of embryonic heart and skeletal muscles, using reverse transcripts PCR amplified with myosin-coding primers. Second, what is the patterns of expression of these sarcomeric myosins? When does each appear, and is each isoform expressed in a single fiber type, or are there overlapping patterns of expression? In situ hybridization to transcript-specific probes will be used to determine the temporal and spatial patterns of expression of each transcript. And third, is the expression of these isoforms regulated by thyroid hormones? Metamorphosis will be manipulated by adding thyroxine to the rearing medium of thyroidectomized or goitrogen treated embryos. In situ hybridization will then be used to assay expression of each isoform in the presence or absence of thyroxine. Together, these studies will provide a more complete understanding of the range of fiber types and their regulation in developing vertebrates.